When partest diffs, make sure the outDir exists
When partest diffs, make sure the outDir exists

Was getting the following in CI, because test/files/neg/t10019-neg.obj
wasn't being created.  The reason it was even going down this route was
because the fatal warnings option was still in a flags file (fix in the
previous commit).

    [error] Uncaught exception when running partest: java.util.concurrent.ExecutionException: java.lang.RuntimeException: Error running /home/travis/build/scala/scala/test/files/neg/t10019.scala
    [error] sbt.ForkMain$ForkError: java.util.concurrent.ExecutionException: java.lang.RuntimeException: Error running /home/travis/build/scala/scala/test/files/neg/t10019.scala
    [error] 	at java.util.concurrent.FutureTask.report(FutureTask.java:122)
    [error] 	at java.util.concurrent.FutureTask.get(FutureTask.java:192)
    [error] 	at scala.tools.partest.nest.AbstractRunner.runTestsForFiles(AbstractRunner.scala:353)
    [error] 	at scala.tools.partest.nest.AbstractRunner.$anonfun$run$17(AbstractRunner.scala:254)
    [error] 	at scala.tools.partest.nest.AbstractRunner.$anonfun$run$17$adapted(AbstractRunner.scala:250)
    [error] 	at scala.collection.ArrayOps$WithFilter.foreach(ArrayOps.scala:73)
    [error] 	at scala.tools.partest.nest.AbstractRunner.$anonfun$run$15(AbstractRunner.scala:250)
    [error] 	at scala.tools.partest.nest.AbstractRunner.run(AbstractRunner.scala:250)
    [error] 	at scala.tools.partest.sbt.PartestTask.execute(Framework.scala:55)
    [error] 	at sbt.ForkMain$Run.lambda$runTest$1(ForkMain.java:304)
    [error] 	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    [error] 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    [error] 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    [error] 	at java.lang.Thread.run(Thread.java:748)
    [error] Caused by: sbt.ForkMain$ForkError: java.lang.RuntimeException: Error running /home/travis/build/scala/scala/test/files/neg/t10019.scala
    [error] 	at scala.tools.partest.nest.AbstractRunner.liftedTree1$1(AbstractRunner.scala:312)
    [error] 	at scala.tools.partest.nest.AbstractRunner.runTest(AbstractRunner.scala:310)
    [error] 	at scala.tools.partest.nest.AbstractRunner.$anonfun$runTestsForFiles$2(AbstractRunner.scala:335)
    [error] 	at scala.tools.partest.package$$anon$2.call(package.scala:141)
    [error] 	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    [error] 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    [error] 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    [error] 	at java.lang.Thread.run(Thread.java:748)
    [error] Caused by: sbt.ForkMain$ForkError: java.io.IOException: No such file or directory
    [error] 	at java.io.UnixFileSystem.createFileExclusively(Native Method)
    [error] 	at java.io.File.createTempFile(File.java:2026)
    [error] 	at scala.tools.partest.nest.Runner.$anonfun$diffIsOk$2(Runner.scala:425)
    [error] 	at scala.tools.partest.nest.Runner.diffIsOk(Runner.scala:425)
    [error] 	at scala.tools.partest.nest.Runner.checked$1(Runner.scala:537)
    [error] 	at scala.tools.partest.nest.Runner.$anonfun$runNegTest$3(Runner.scala:540)
    [error] 	at scala.tools.partest.nest.Runner.$anonfun$runNegTest$1(Runner.scala:540)
    [error] 	at scala.tools.partest.nest.Runner.runInContext(Runner.scala:437)
    [error] 	at scala.tools.partest.nest.Runner.runNegTest(Runner.scala:532)
    [error] 	at scala.tools.partest.nest.Runner.run(Runner.scala:631)
    [error] 	at scala.tools.partest.nest.AbstractRunner.liftedTree1$1(AbstractRunner.scala:310)
    [error] 	at scala.tools.partest.nest.AbstractRunner.runTest(AbstractRunner.scala:310)
    [error] 	at scala.tools.partest.nest.AbstractRunner.$anonfun$runTestsForFiles$2(AbstractRunner.scala:335)
    [error] 	at scala.tools.partest.package$$anon$2.call(package.scala:141)
    [error] 	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    [error] 	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    [error] 	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    [error] 	at java.lang.Thread.run(Thread.java:748)

scala/scala,lrytz/scala,scala/scala,lrytz/scala,scala/scala,scala/scala,scala/scala,lrytz/scala,lrytz/scala,lrytz/scala,scala/scala,lrytz/scala